1. Field of the Invention
The present invention relates to an electrical resistance heating element.
2. Description of the Related Art
Heating elements of the kind described herein are intended for use in heat treatment and sintering processes, in inert and reducing atmospheres, and also oxidizing atmospheres and vacuum conditions, up to extremely high temperatures, such as temperatures as high as 2300° C., but also at low temperatures, e.g., temperatures of 500° C.
Resistance heating elements of the present kind are manufactured by applicants. The resistance heating elements are of a widely varying form and are based on NiCr, FeCrAl, SiC, MoSi2, and alloys of those materials. Those materials are used in a plurality of atmospheres and at different temperatures. Heating elements that are composed mainly of Mo, W, Ta (tantalum), and graphite are used at temperatures around and above 2000° C. In the case of lower temperatures a molybdenum silicide and aluminum oxide composite material is used.
The heating elements include one, two, or more legs, as well as two terminals for connection to a source of electric current. The diameter of the terminals is greater than the diameter of the glow zones of the elements, to reduce the amount of heat generated at the terminals. The elements are in the form of homogenous rods through which electric current flows.
There is a desire to increase the electrical resistance in the glow zone of the element to obtain the same element temperature at a lower current strength, which would greatly lower the power supply operating costs of the elements.
The solution in which the element is provided with a smaller outer diameter, and therewith a higher electrical resistance, results in a smaller element radiation surface, which is highly disadvantageous since a larger radiation gives a larger heat yield through radiation heat. Moreover, thin elements result in mechanical strength problems at high temperatures.
Such desirable attributes are fulfilled by the present invention.